The present exemplary embodiment relates to light emitting diodes (LEDs). It finds particular application in conjunction with control of the intensity and uniformity of LEDs by indentifying failure conditions, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
In many lighting applications, LEDs are selected to provide illumination instead of an incandescent or other conventional illumination sources. Generally, the LEDs are arranged in clusters or arrays to coincide with particular design specifications to provide a desired light output. Because the voltage versus current characteristics of the LED are much like any diode (that is, current approximately an exponential function of voltage), a small voltage change results in a huge change in current. Added to deviations in the process, this means that a voltage source may barely make one LED light while taking another of the same type beyond its maximum ratings and potentially destroying it.
LED clusters can have requirements for intensity and pattern of light output. In general, LED clusters are built with two or more chains of two or more LEDs in series. Each chain can be driven by a power circuit that provides current to each of the LEDs contained therein. In order to meet desired output requirements, it is important to monitor and control the luminous intensity and the light uniformity by supervising each LED chain.
Conventionally, sense resistors are employed to detect current drawn by one or more LED clusters. As known in the art, current drawn by each LED cluster is related to the output and operating status (e.g., open circuit, short circuit, etc.) of the LEDs. Since light output is dependent on proper current, identification of failure down to a single LED in a cluster is desired. However, such a solution can affect overall system efficiency due to significant power dissipation caused by using such sense resistors.
This task becomes even more important when fewer LEDs are utilized in a cluster, wherein a single LED failure can cause luminous intensity and light uniformity that is outside desired parameters. In such applications, the relative power savings of a cluster with a minimal amount of LEDs can be greatly affected by the use of large current consumption devices such as inline sense resistors.
In order to mitigate this effect, the sense resistors must be relatively large to minimize unnecessary power consumption, and is therefore typically external to the LED power circuit. However, this external placement can mandate that the packaging for the LED power circuit includes additional pins to enable measurement of the voltage across the sense resistor. The resulting increase in pin count can preclude the use of smaller, more desirable chip packaging for conventional power circuits.
Accordingly, there is a need for more non-invasive systems and methods to detect single LED failure within an LED cluster.